Fig. 1.8 illustrates the concept of osmosis and the measurement of the osmotic pressure of a solution.
Osmotic pressure is determined by the number of solute molecules dissolved in the solution. It is not dependent on such factors as the size of the molecules, their mass, or their chemical nature (e.g., valence). Osmotic pressure (π), measured in atmospheres (atm), is calculated by van’t Hoff’s law as follows:
Equation 1.7 ˜= nCRT,
CHAPTER 1 Principles of Cell and Membrane Function
K+ Na+ 3Na+ Na+, K+ -ATPase 3Na+-Ca++ antiporter IntracellularconcentrationsExtracellularconcentrations3Na+ Na+: 145 mEq/L K+: 4 mEq/L Glucose: 5 mmol/L Ca++: 2.5 mEq/L (ionized) Na+: 12 mEq/L K+: 120 mEq/L Glucose: 2 mmol/L Ca++: 0.001 mEq/L (ionized) Ca++ Secondary active transport of Ca++ Primary active transport of Na+ and K+ Passive transport Na+ channel K+ channel Glucose uniporter Glucose 2K+ ATP •Fig. 1.7 Examplesofseveralmembranetransporters,illustratingprimaryactive,passive,andsecondaryactivetransport.Seetextfordetails.ATP,adenosinetriphosphate.
Glucoseistransportedbytheepithelialcellsthatlinethegastrointestinaltract(smallintestine),andbycellsthatformtheproximaltubulesofthekidneys.Inthegastrointestinaltract,theglucoseisabsorbedfromingestedfood.Inthekidney,theproximaltubulereabsorbstheglucosethatwasfilteredacrosstheglomerularcapillariesandtherebypreventsitfrombeinglostintheurine.Theuptakeofglucoseintotheepithelialcellfromthelumenofthesmallintestineandfromthelumenoftheproximaltubuleisasecondaryactiveprocessinvolvingthesodium-glucose–linkedtransportersSGLT-1andSGLT-2.SGLT-2transportsoneglucosemoleculewithoneNa+ ion,andtheenergyintheelectrochemicalgradientforNa+ (intothecell)drivesthesecondaryactiveuptakeofglucose.Accordingtothefollowingequation,forcalculatingtheelectrochemicalgradient,andifthemembranepotential(Vm)is−60mVandthereisa10-fold[Na+]gradientacrossthemembrane,anapproximate100-foldglucosegradientcouldbegeneratedbySGLT-2:[Glucose]i [Na+ ]o V 615. mV
Thus,iftheintracellularglucoseconcentrationwas2mmol/L,thecellcouldlowertheextracellularglucoseconcentrationtoapproximately0.02mmol/L.However,byincreasingthenumberofNa+ ionstransportedwithglucosefromonetotwo,SGLT-1cangenerateanearly10,000-foldglucosegradient: 615. mV
Again,iftheintracellularglucoseconcentrationis2mmol/L,SGLT-1couldremovevirtuallyallglucosefromeitherthelumenofthesmallintestineorthelumenoftheproximaltubule(i.e.,theluminalglucoseconcentration≅ 0.0002mmol/L).
where molecules at 37°C can exert an osmotic pressure of 2.54 n = number of dissociable particles per molecule × 10−2 atm, as calculated with eq. 1.7 and the following C = total solute concentration values: R = gas constant n = 1 T = temperature in degrees Kelvin C = 0.001 mol/L